Related to a mounting structure of the spiral spring of the seat reclining device, a reclining device is disclosed in, for example, JP2001-292850A, which includes a first hook plate 102 fixed at seat back frame 101, a second hook plate 104 fixed at a seat cushion frame 103, and a spiral spring 105 for applying a biasing force to the seat back frame 101 to rotate forward. The spiral spring 105 includes an inner end portion 105a formed by bending an end of the spiral spring 105 inward in a radial direction of the spiral spring 105 and an outer end portion 105b formed by bending an end of the spiral spring 105 outward in a radial direction of the spiral spring 105, and the spiral spring 105 applies the biasing force in a manner where the inner end portion 105a engages the first hook plate 102, and an outer end portion 105b engages the second hook plate 104.
The first hook plate 102 includes a first spring hook 102a formed so as to protrude in a direction that parallels to a rotation shaft of the seat back frame 101, and the first spring hook 102a engages the inner end portion 105a of the spiral spring 105 in a manner where the inner end portion 105a is hooked-on an end portion of the first spring hook 102a. 
The second hook plate 104 includes a second spring hook 104a formed so as to protrude in a direction that parallels to a rotation shaft of the seat back frame 101, and the second spring hook 104a engages the outer end portion 105b of the spiral spring 105 in a manner where the outer end portion 105b is hooked on an end portion of the second spring hook 104a. 
In this configuration, because the rotations of the inner end portion 105a and the outer end portion 105b relative to the first spring hook 102a or the second spring hook 104a are not limited, a portion near the outer end portion 105b of the spiral spring 105 which is comprised of a wire 106 moves inward in a normal direction thereof because of a spring force of the wire 106 to extend to be straight, and a portion near the inner end portion 105a of the wire 106 moves outward in a normal direction thereof.
Thus, when the wire 106 is unevenly provided, an internal stress applied to each part of the wire 106 becomes uneven. In other words, at a portion of the wire 106, where its curvature is relatively large, a level of an internal stress becomes high, and at other portions of the wire 105, where its curvature is small, a level of an internal stress becomes low.
In this configuration, the spiral spring 105 needs to be designed in a manner where a working stress thereof is set on the basis of the highest internal stress. To provide the spiral spring 105 having such structure, the spiral spring 105 having a high working stress has been used, however, a cost of such spiral spring 105 is relatively high, and a diametral dimension of the spiral spring 105 becomes relatively large.
A need thus exists to provide a seat reclining device by which chances that the position of the wire configuring the spiral spring is unevenly provided are reduced, and an internal stress applied to each part of the wire configuring the spiral spring can be equalized.